The first generation of high temperature electrolyzer cells or high temperature fuel cells, comprised a support formed by the electrolyte and was thus designated as an Electrolyte-Supported Cell (<<ESC>>). Such an electrolyte-supported cell is illustrated in FIG. 1: the oxygen O2 electrode (1) and the hydrogen or water electrode (2) are positioned on either side of the thick electrolyte which forms the support (3).
The second generation of high temperature electrolyzer cells or high temperature fuel cells comprised a support formed by an electrode and was thus designated as an Anode-Supported Cell (<<ASC>>) in <<SOFC>> terminology or Cathode-Supported Cell (<<CSC>>) in <<HTE>> terminology. Such an electrode supported cell <<ASC>> or <<CSC>> is illustrated in FIG. 2: the electrolyte (3) and the oxygen electrode (1) are positioned on the thick hydrogen or water electrode (2) which is used as a support.
The third generation of high temperature electrolyzer cells or high temperature fuel cells, in which we are more particularly interested herein comprises a porous metal support and is therefore designated as a Metal-Supported Cell (<<MSC>>). Such a metal-supported cell may appear according to two configurations which are respectively illustrated in FIGS. 3A and 3B depending on whether the electrode which is placed in contact with the porous metal support is the hydrogen or water electrode (2) (FIG. 3A) or else the oxygen electrode (1) (FIG. 3B). More details on these various types of <<HTE>> and of <<SOFC>> may be found in document [1].
The metal-supported cells illustrated in FIGS. 3A and 3B include four layers (including one metal layer and three ceramic layers), i.e.:                the porous metal support (4), generally with a thickness of less than 1 mm which ensures:                    the mechanical support of the cell by its mechanical properties and its thickness,            the distribution of the gases as far as (up to) the electrode with view to electrochemical reactions by means of its porosity,            the collection of the current by its conducting metal nature.                        The H2/H2O electrode (2) which is the anode for a SOFC and the cathode for a HTE. By means of the metal support (4), this electrode may be made thinner, with for example a thickness of less than 50 μm, its resistance to redox cycles is thus better and its cost is lower;        the electrolyte (3), an ion conductor for O2− ions. The electrolyte (3) may be made thinner, with for example a thickness of less than 10 μm, its operating temperature may thus be lowered;        the O2 electrode (1) which is the cathode for a SOFC, and the anode for a HTE. This electrode (1) may be made thinner with for example a thickness of less than 50 μm.        
In documents [2] and [3] different types of metal materials are mentioned for elaborating porous metal supports. First of all, these are metal alloys produced by standard, conventional, metallurgy, and then alloys produced by powder metallurgy, which are presented in these documents as being better candidates for producing the metal support [2-3].
On these supports are deposited ceramic layers (anode, electrolyte, cathode) by a vacuum plasma deposition method (VPS, or Vacuum Plasma Spraying), which does not require any sintering step at a high temperature [2-3].
The method for making the porous material is not described in these documents, and optimization of the microstructure and of the porosity with optionally a gradient of the latter, is not either mentioned.
Further, partial oxidation of the porous metal support is neither described nor suggested in documents [2] and [3].
In documents [5] to [8], a porous metal support is described. Two options are contemplated for subsequently depositing the ceramic layers on this support:                either the ceramic layers are deposited on the <<green>> metal support, i.e. not sintered, and the assembly of the support and of the ceramic layers is then co-sintered at high temperature, but under a reducing atmosphere for avoiding significant oxidation of the metal support;        or the ceramic layers are deposited on the already sintered metal support and are then sintered independently, which should be accomplished at a lower temperature, doubtless for avoiding oxidation of the metal and its densification.        
The techniques for depositing ceramic layers are in majority conventional wet techniques such as strip casting or screen printing.
The porous metal support may, according to document [4], be produced by strip casting [4].
Document [8] reports a tubular porous metal support made by isostatic pressing, doubtless because the tubular geometry does not allow strip casting.
Documents [4] to [8] do not mention any optimization of the microstructure, or of the porosity with optionally a gradient regardless of the technique used. Documents [4] to [8] do not either mention any preliminary partial oxidation step (pre-oxidation) of the porous metal support before its use.
In documents [9], [10], and [11], a two-zone metal support is used which is dense on the sides for ensuring tightness and pierced in the middle for distributing the gases.
The holes in the central portion of the support are made by machining (photochemical [10] or laser [11]) machining). The holes formed have a diameter of 10 to 30 μm. Document [9] proposes a cell structure for this porous portion of the support.
Because of the technology used for producing the holes, the size of the holes is identical, without any gradient, over the whole thickness of the metal support and there is no optimization of the microstructure.
An oxidation of the metal support prior to its use (pre-oxidation) is neither mentioned nor suggested in these documents.
The ceramic layers as for them are deposited by wet techniques.
The range of operating temperatures aimed in documents [9], [10], and [11] is only from 500° C. to 600° C.
Document [12] relates to tubular metal supported cells. It is simply indicated that the supporting tube made of a porous metal material is produced by <<techniques with an industrial cost>>, but without further information.
Document [13] mentions metal-supported cells, the porous metal support of which consists of metal (<<Hastelloy>>) plates with a porosity of 27.5% [13]. It would seem that these are rather plates pierced with holes of the type of those described in documents [9], [10], and [11].
Document [14] relates to tubular metal supported cells, wherein the metal support is porous with porosity between 20 and 75%. This metal support is prepared by a wet process and there is no mention of optimizing its porosity and even less of a gradient of the latter, or of any oxidation of the metal support before its use (pre-oxidation).
In document [15], a concept called <<bipolar plate-supported SOFC>> is discussed. A metal plate, which acts as an interconnector (or bipolar plate) between two adjacent cells of a stack, is also used as a support for ceramic cells. Between this dense plate and the ceramic cells, porous metal materials are inserted. They are deposited by wet techniques just as for the ceramic layers and are co-sintered with the latter [15]. There is no mention is this document [15] of any optimization of the porosity of the metal porous materials, and even less of any gradient of the latter, or of any oxidation of these metal porous materials prior to their use.
Document [1] already cited, mentions cells with a metal support made of a FeCr alloy, but no detail on this type of support and especially its porosity, as well as on its forming, shaping, is provided.
Document [16] relates to a metal-supported cell, the metal support of which is a metal plate having cavities and channels for distributing the gases as far as (up to), the electrode, these channels being made by chemical etching. So really, one cannot speak of porosity in this document and even less of an optimization or of a gradient of the latter. Further, no oxidation of the metal support prior to its use (pre-oxidation) is mentioned in this document.
Moreover it will be noted that in all the documents cited above, the <<HTE>> application is seldom or even never mentioned.
However, document [17] describes a method for preparing a reversible SOFC, i.e. which may operate in an SOFC mode or in a HTE mode. In this document, the use of a porous metal support especially made of ferritic stainless steel is mentioned. The porosity of the metal support is achieved by adding pre-forming agents during the manufacturing of the support and the porosity may be finely adjusted by acting on the amount of added pre-forming agents. However, there is no mention in this document of pore size or of porosity gradient. Further, in the examples of this document, it is specified that the porous support is produced by strip casting.
Finally, an oxidation of the porous support prior to its use (pre-oxidation) is neither mentioned nor suggested in this document.
The document of Molins et al. [18] presents a porous metal material, produced by pressing-sintering under hydrogen starting with the stainless steel commercial grade 430L and evaluates its resistance to oxidation. A porosity of 40% is indicated, and the conclusion is drawn that the resistance to oxidation of this porous material is not satisfactory for an SOFC application. This document does not mention the deposition of ceramic layers on this porous material, nor the optimization of microstructure and in particular of its porosity, nor the oxidation of the porous material prior to its use, i.e. the pre-oxidation of the porous material.
In none of documents cited above, is mentioned the question of optimizing the porous metal support for promoting adherence, anchoring of the ceramic layers, while this is a crucial problem for proper operation of an SOFC-HTE cell.
Therefore there exists a need for a porous metal support for a high temperature electrolyzer cell (<<HTE>>) or a high temperature fuel cell (<<SOFC>> or <<Solid Oxide Fuel Cell>>) which allows excellent adherence, anchoring of the ceramic layers and further excellent resistance to oxidation by the gases during use.
More generally there exists a need for such a porous metal support which has properties meeting the following criteria and requirements:
Physical and Physico-Chemical Properties:
1°) the support should play the role of a mechanical support for the cell: it should therefore have some cohesion and sufficient thickness, as well as sufficient mechanical properties;
2°) the support should ensure delivery and distribution of the gases as far as the electrode: it should therefore have a porosity adapted to the contemplated gases and flow rates;
3°) the support should ensure collection of the current:                it should therefore be an electron conductor, which is possible because of its metal nature,        it should remain an electron conductor over time, during operation for a long time of the cell at a high temperature, i.e. it should especially resist oxidation in the relevant atmospheres, i.e. H2/H2O or O2/air depending on the selected configuration;        
4°) the support should allow deposition of the first electrode, which is a ceramic material (oxide), or a metal/oxide cermet (typically Ni-YSZ), it should therefore:                have a surface which allows both good physical and chemical adherence, anchoring of this ceramic or cermet layer;        be able to withstand the sintering step of the ceramic layers of the electrodes and of the electrolyte which may be required according to the selected methods for depositing ceramic layers. In order to be able to withstand this sintering step, the support should:                    retain its porosity during this treatment,            not be significantly oxidized.                        have a coefficient of thermal expansion compatible with that of the deposited layers of ceramics or cermet;        not chemically react with the deposited electrode material (oxide or cermet).Economic Properties:        
5°) the support should be inexpensive, one of the goals being to reduce the cost of metal-supported cells as compared with other types of cells;
6°) the support should be able to be formed, shaped, with simple, fast, robust and not very expensive techniques;
7°) the support should be able to be formed, shaped, with various sizes and shapes which may be required for the application (circular, square shapes, small sizes, large sizes . . . ).
The goal of the present invention is to provide a porous metal support for a high temperature electrolyte cell (<<HTE>>) or high temperature fuel cell (SOFC or Solid Oxide Fuel Cell) which i.a. meets the needs mentioned above, which has the properties mentioned above, and which meets the criteria and requirements listed in the foregoing.
The goal of the present invention is further to provide such a porous metal support which does not have the drawbacks, defects, limitations and disadvantages of the metal porous substrates of the prior art, especially illustrated by the documents mentioned above, and which overcomes the problems of the porous metal supports of the prior art.